This invention relates to the field of nuclear reactors for nuclear steam supply systems and is particularly adapted for use in a pressured water reactor (PWR) having control rod drive mechanism upper pressure housings extending above the reactor closure head.
During a refueling or maintenance outage, the reactor vessel requires venting of air during reactor coolant filling and vessel pressurization. This is done through vents at the top of the control element drive mechanism (CEDM) upper pressure housings. The originally provided vent assembly for this purpose consists of a housing which includes a central passageway containing a flow blocking ball, and seat therefor, and an internally threaded portion downstream and outwardly therefrom. A vent stem with a mating externally threaded hollow body and with torquing pads formed by cross-slotting for engagement by a torquing tool on its downstream end is normally threadedly assembled to hold the ball against its seat to seal the housing passageway. An externally threaded housing closure plug closes the downstream portion of the housing passageway and encloses the downstream vent stem end.
It is important to vent properly after refueling to minimize the occurrence of stress corrosion cracking on the PWR control rod drive mechanism housing. Utilizing the prior art venting structure described above, however, requires about 20-30 minutes per drive mechanism, and involves disassembly of the ball seal housing to replace o-rings. During venting operations, the threaded plug is removed, a special vent stem hollow torquing tool inserted in the cross-slotting, and the vent stem unscrewed slightly to allow air to pass through the vent stem and up the tool. Afterward, the stem is tightened, the tool removed and the threaded plug reinstalled.
This standard venting process is very time consuming and creates significant manrem exposure during critical path restart activities. Also, the standard threaded plug has an affixed o-ring seal in case the operator wishes to weld the plug to the housing. If the ball (primary seal) is leaking, the o-ring (not intended to be a primary seal) may contain the leakage and it will not be realized that the primary seal is leaking.